In electrorefining (ER) and electrowinning (EW) electrodes are immersed in an electrolyte and an electric current is passed between them. The anode is made positive and the cathode made negative so that an electric current passes through the electrolyte from anode to cathode.
In electrorefining (ER), the metal anode is soluble. That is to say that the metal enters into the electrolyte under the influence of the potential between the anode and cathode. For example, in the electrorefining of copper, the anode is made of copper and the copper enters the electrolyte from the anode. The metal, now in the electrolyte, is transported through the electrolyte to the cathode where it is deposited. The cathode may be of the same metal as the metal that is being deposited or it may be of a different metal. For example, in the electrorefining of copper it was at one time common to employ a cathode made of copper. However, a stainless steel cathode is now commonly employed which quickly becomes coated with copper and which from then on essentially performs as a copper cathode. The deposited copper is mechanically removed from the stainless steel cathode and the cathode reused. The copper deposited on the cathode is highly pure. Impurities which were in the anode metal fall out as a solid as the anode is dissolved and may contain useful by-products, for example, gold. Besides copper, metals purified by ER include gold, silver, lead, cobalt, nickel, tin and other metals.
Electrowinning (EW) differs from electrorefining in that the metal sought is imported into the cells via, for example, pipelines, and is already contained within the electrolyte. In the example of copper, sulfuric acid is typically employed to dissolve copper from an oxide form of copper ore and the resulting liquid, after concentration, is imported into an electrowinning cell to have the copper extracted. An anode and cathode are immersed in the electrolyte and a current is passed between them, again with the anode being positive and the cathode being negative. In electrowinning, the anode is not soluble but is made of an inert material. Typically, a lead alloy anode is used in the case of copper. The cathode may be of the same metal that is being extracted from the electrolyte or it may be of a different material. For example, in the case of copper, copper cathodes may be used although stainless steel cathodes are commonly employed which quickly become coated in copper. Under of the influence of the electric current, the metal to be won leaves the electrolyte solution and is deposited in a very pure form on the cathode. The electrolyte is changed by this process having given up a large proportion of its metal content. Besides copper, metals obtained by electrowinning include lead, gold, silver, zinc, chromium, cobalt, manganese, aluminum and other metals. For some metals, such as aluminum, the electrolyte is a molten material rather than an aqueous solution.
As an example of the voltages and current involved, in copper refining, the cell voltage is generally about 0.3V and in copper electrowinning is about 2.0V. In both cases the current density is about 300 A/m2 and the area of each electrode at present is about 1 m2. These figures differ considerably for different metals and widely varying current densities may be used for the same metal but the invention applies to the electrorefining and electrowinning of all metals. In ER and EW the starting point is an anode juxtaposed to a cathode in an electrolyte contained in a tank. The anodes and cathodes may be plates which hang from a supporting hanger bar. The plates may also have protrusions or lugs on both sides of the plates, which enable the plates to be hung from supporting bars, for example, power supply busbars. But many cathode plates and many anode plates may be used, interleaved, with all the anode plates connected in parallel and all the cathode plates connected in parallel contained within a single tank of electrolyte. Electrically this still looks like a single cell and in the industry it is therefore commonly called a cell. In the ER and EW industry, “cell” is almost universally used to mean a tank filled with anodes and cathodes in parallel. In the ER and EW industry, “tank” can mean the same as “cell”, above, or it can mean the vessel alone, depending on the context. In tankhouses “tanks” are connected in series. A typical ER tankhouse might therefore require an electrical supply of the order of 36,000 Amps at 200 Volts.
Reference is now made to FIG. 1, which illustrates a typical tankhouse in prior art. A tankhouse 100 comprises a plurality of tanks such as tanks 110, 120 and 130. Each tank comprises one cell. A cell is composed of many cathodes such as cathodes 126A and 126B in parallel and many anodes such as anodes 124A and 124B in parallel. The cathodes in tank 110 are connected to busbar 112. The anodes in tank 120 are connected to busbar 122. Busbars are connected to a power supply such as power supply 102. An anode busbar of tank 110 is connected to a positive terminal 104 of power supply 102, whereas a cathode busbar of tank 130 is connected to a negative terminal 106 of power supply 102. In FIG. 1 tanks 110, 120 and 130 are connected in series so that cathode busbars and anode busbars are connected in adjacent tanks such as tanks 110 and 120. Power supply 102 acts as a Direct Current (DC) voltage source and the DC voltage source is connected across the series circuit formed by tanks such as tanks 110, 120 and 130 to force the desired current through tanks such as tanks 110, 120 and 130. The total current is maintained at a desired value. Ideally, the current should divide equally between the cathodes such as cathodes 126A and 126B. In practice, there is significant variation in the resistance of each cathode-current path and hence there is variation in the value of the individual cathode currents. This means in practice that the metal production process operates below optimum efficiency. Further, there may be sometimes a disruption to the operation of part of a cell when a short circuit develops between an anode plate and a cathode plate such as, for example, anode 124A and cathode 126A. This is typically due to a bump or spike of metal growing on a cathode plate and increasing in size until it connects the cathode plate with an adjacent anode plate. The spike of metal has to be physically removed to permit normal operation of the cell to continue. Another disruption to normal production can occur when an individual cathode or individual anode becomes disconnected from the electrical circuit.
Reference is now made to FIG. 2, which illustrates a cross section of an electrolysis cell 200 comprising a blade 212, which may be an anode blade or a cathode blade, in prior art. Blade 212 is immersed in electrolyte 202. As illustrated in FIG. 2, an electrical connection to blade 212 is made through protrusions or lugs on both sides of blade 212 on the upper side of blade 212. Blade 212 may comprise or be connected to a hanger bar 210. The protrusions on both sides may also be seen to form hanger bar 210. On the right hand side in FIG. 2, right side of hanger bar 210 rests on a busbar 220 so that there is a contact area 222 between busbar 220 and hanger bar 210. Contact area 222 causes busbar 220 and hanger bar 210 to form a part of an electrical circuit (not shown). A disconnection of contact area 222 is typically caused by corrosion or burning of contact area 222 or by a foreign obstacle becoming jammed between hanger bar 210 and busbar 220. On the left hand side in FIG. 2, left side of busbar 220 rests on an insulated supporting bar 230. Supporting bar 230 may also be a busbar so that the electrode 212 is electrically connected through two paths so as to reduce the effect of a bad contact to one of the sides of hanger bar 210. A short circuit results in an unusually large amount of current flowing in cathode 126A and anode 124A shorted together. Methods conventionally employed to detect short circuits are less than ideal.
One method is to detect the overheating of the electrodes resulting from the short circuit. This is less than satisfactory because damage to the electrode, the hanger bar of the electrode or the busbar connected to the hanger bar may have resulted before the short is detected.
This method will become even less acceptable as new, expensive, high-performance anodes, are introduced into electrowinning processes. In electrowinning, inert lead anodes have been commonly used. In recent years coated titanium anodes have been increasingly adopted because of their superior properties. However, the coated titanium anodes are more expensive than lead anodes and more readily damaged by heat. It has therefore become imperative that these high-value electrodes should be protected against damage.
The problem in prior art solutions is that electrodes are not sufficiently protected from damage and that electrodes are not handled separately to determine electrical currents in individual electrodes.